The most primitive form of irrigation control involves a decision by an operator to begin or discontinue application of irrigation water. The effects of under-irrigation are apparent in that soil and plant conditions suffer. However, in many cases excessive irrigation does not cause apparent harm to plants, trees, and shrubs. Where there is an abundance of water, a major harm is waste of irrigation water and energy required for its distribution. Additionally, there may be significant but subtle impairment of plant growth.
Several methods have been developed to improve the efficiency of irrigation while minimizing the burden on human operators. One technique involves the use of pre-programmed timers to automatically start and terminate irrigation. Such timer systems have been refined to provide for multiple timing cycles which vary in accordance with plant growth cycles. However, such systems are limited in that there is no feedback between actual plant water consumption and water application. For example, timer systems are unable to reduce water application during times of humid or rainy weather, and do not account for variable water requirements or terrain topography. Because of water run-off, irrigating a sloping yard, the side of a hill, or a berm is quite difficult and costly. Conversely, less water tends to be required when irrigating depressed or low terrain.
All agricultural experts recommend that to properly irrigate, an operator must plug or use a shovel to dig down to root level to see if watering was too little or too much or if watering was needed.
Because of these limitations, systems which measure actual soil moisture content were devised. Such systems measure soil moisture, including soil electrical conductivity, in a variety of ways. Typical of systems developed which utilize various types of buried sensors to determine when the soil requires irrigation are those found in U.S. Pat. Nos. 2,812,976; 2,611,643; 3,961,753; 4,197,866; and 4,545,396. In general, these systems utilize an inground sensor or probe which will activate the irrigation process dependent upon a certain indicated moisture level and timers.
Efforts to overcome these problems have involved the combining of timers with soil moisture sensors. In such systems, the timers actuate irrigation at predetermined times, but only if the moisture soil sensors indicate that the soil has become dry. Timers eliminate the danger of over-watering while waiting for moisture to penetrate to the sensors. Still, they are not responsive to changes in weather, nor can they sense any characteristics, such as variable terrain topography, other than soil moisture.
Other automatic irrigation systems which have sensed the moisture level in the soil of the area to be irrigated have no way of determining or adequately controlling the quantity of water delivered to the area. Therefore, they cannot provide the capability of conserving limited water resources.
Another problem left unsolved by previous automated approaches is the relatively high cost of having a remote sensor associated with each sprinkler head.